The present invention relates to cam followers or tappets for the valve gear of an internal combustion engine, and particularly relates to cam followers of the type having a roller or wheel provided on the end thereof for rolling contact with the surface of a rotating lobe on the engine cam shaft. Tappets of the aforementioned type are known in the internal combustion engine as roller followers and are employed to reduce friction between the cam shaft and the cam follower for the purpose of increasing the power output of the engine, i.e. reducing friction losses and to improve the wear life of the cam shaft and cam follower.
It has been found desirable in some instances to retrofit existing production internal combustion engines, and particularly engines for mass produced automotive applications, with roller follower tappets where the engine was originally designed and manufactured with tappets having the ends thereof in direct frictional sliding engagement with the lobe of the cam shaft. Where it is desired to retrofit an existing production automotive engine with roller follower tappets, it is desired to do so without otherwise altering the engine block to minimize changes in the remaining valve gear components. In order to accomplish such a retrofit of roller follower tappets, it is necessary to incorporate the roller on the end of a tappet substantially within the dimensional envelope of the existing engine tappet to eliminate the necessity of relocating the camshaft and minimize alterations to other valve gear components. In order to achieve such an incorporation of a roller on the end of an existing tappet substantially within the original tappet envelope, it has been found economical to provide a slot or recess in the end of the tappet in a manner so as to form a pair of spaced parallel support portions extending from the end of the tappet, and to provide cross holes for receiving therein the axle of the roller with the roller nestled between the parallel support portions. The roller has a diameter sufficiently great so that the peripheral surface thereof extends beyond the end of the tappet. Thus, the roller will contact the cam lobe and roll thereon preventing contact of the end of the tappet portions with the cam.
Where an existing production engine tappet incorporates therein internal hydraulic lash adjusting means, the material and the construction of the tappet body are dictated by the requirements of the tappet envelope, clearance and wear requirements. In particular, in certain engine valve gear, there is a requirement for a close sliding fit between the tappet and the engine guide bore to prevent loss of engine oil which is pressure-fed from an engine oil port in the tappet guide bore to a collector ring on the surface of the tappet for flow through a port to the interior of the tappet. It has been found that compatible iron alloy materials are satisfactory and in particular, medium and low carbon steels are suitable materials for the tappet body in order to provide a wear surface that will maintain the proper clearance between the tappet and the guide bore in the engine and also provide the required wear life in engine service.
Thus, where roller followers are to be retrofitted into the valve gear of an existing production engine, it has been found difficult to provide a recess in the end of the tappet for receiving the roller therein and yet provide sufficient material for support of and retention of the roller axle in the body portions which form the sides of the roller receiving recess and to provide such axle support and yet permit the use of relatively low cost materials. For example, prior attempts to incorporate a roller in the end of a hydraulic tappet compatible for use in an existing iron block production engine have resulted in fracture of the tappet body material in the region about the apertures formed in the support portions for receiving the transversely disposed roller axle.
In particular, problems have been experienced in retaining the axle for a roller follower in the end of a hydraulic tappet where the ends of the axle are deformed, by techniques such as orbital staking, for retaining the axle securely in the end of the tappet. Previously, it has been found necessary to ream and hone the transversely formed aperture in the support portions of the tappet for providing a precision fit with the roller axle in order that, upon deformation of the axle material, sufficient retaining forces are developed in the aperture by the deformation of the axle material. This reaming and honing of the transverse axle receiving holes in the tappet has proven to be prohibitively costly for mass production of roller follower hydraulic tappets for automotive engines. Furthermore, even where reaming and honing have been employed to ensure a precision fit of the axle before orbital staking, it has been found that after staking sufficient stresses have been developed in the tappet body arm portions that fracturing has occurred.